> The cars know where they are

not if they're in a tunnel or a highrise canyon, or there is weather-related interference, or the maps have not been updated to reflect reality, or there is a detour for construction, etc.

GPS not nearly that reliable. There's a reason drones are moving towards computer vision rather than purely relying on GPS. The physical signs won't glitch out and show 100m offset because a tall building was reflecting GPS signals.

Maybe in a 100% automated system we would replace the icons with QR codes or something but physical markers will likely be here to stay.